Stainless Steel Strip Coated with a Metallic Layer

ABSTRACT

A coated high strength stainless steel strip product with a dense and evenly distributed metallic layer on one side or both sides of said strip is provided. Said layer consists of essentially pure gold, copper, nickel, cobalt, molybdenum, silver, tin or tungsten or alloys of at least 2 of these metals, the thickness of said layer is preferably maximally 15 μm, the tolerance of said layer is maximally +/−30% of the layer thickness, the Cr content of the steel strip substrate is at least 10%, and that the layer has such a good adhesion so that the coated steel strip can be uniaxially stretched to fracture by tensile testing without showing any tendency to peeling, flaking or the like. The metal-coated strip product is suitable for use in applications that are load carrying and is able to transfer electrical currents to a contacting surface without an electrical conductivity drop at the interface between the surfaces.

The present invention relates to a method for manufacturing a metalcoated steel strip product in a roll-to-roll process and in particularto a coated metallic substrate material suitable for manufacturing highstrength stainless steel products. This is achieved by coating ametallic strip with an electrically conductive layer, in accordance withclaim 1.

BACKGROUND OF THE INVENTION

In many electronic devices such as telephones, remote controls,computers, etc., springs and other formed metallic parts are used fordifferent functions. Hence, for electromagnetic shielding (EMS)purposes, springs are used in so called “finger stocks” as gaskets inremovable sections in shielded boxes. In most such products, severaldifferent requirements on the material are at hand. For springs, therequirements are in general related to the mechanical behavior such asforce, relaxation resistance, and fatigue resistance. However, asforming is generally involved, the material must be able to be formed torequested shapes without any cracking. Further, the ongoingminiaturization within this field also puts increasing demands on tightgeometrical tolerances of components and parts in electronic devices. Inaddition to the above, it is sometimes crucial to have well definedelectrical characteristics of such parts and components. This mayinvolve specific properties regarding electrical conductivity or contactresistivity at interfaces within devices. Generally, when suchrequirements are present, the solution is to choose a conductivematerial such as copper or copper alloys, or alternatively to coat asteel with a conductive layer. Copper and copper alloys are oftencharacterized by good electrical conductivity and good formability butmost of them are suffering from low mechanical strength, which meansthat they are not suitable for applications that are highly stressed, asfor instance springs. Alloys of copper and beryllium may be hardened toa tensile strength up to approximately 1400 MPa but also this tensilestrength level limits the spring force, fatigue and relaxationresistance that can be achieved for spring applications. Further,beryllium is a toxic metal, which may put restrictions duringmanufacturing and use due to health considerations. Finally,copper-beryllium alloys are costly and, therefore, less expensiveproducts are requested in many applications.

Coating may be carried out by various methods that can be divided intomechanical and chemical methods. These may also be sub-divided into highand low temperature methods. Mechanical methods may be exemplified bycladding, thermal by spraying or painting. In this context, cladding isrepresented by roll bonding, i.e., to bind two (or more) differentmaterials by a rolling process that is relatively simple and may becarried out with different combinations of substrates and coatings.However, cladding suffers from some technical disadvantages, which arerelated to thickness tolerances and poor adhesion of the coated layer.This often requires a post-bonding heat treatment in order to obtain adiffusion zone between layers. If one (or several) of the layers is/arestainless steel, then a good adhesion is even more difficult to obtaindue to the passive film at the stainless surface. Further, roll bondingis a low speed process and is limited in the possible combinations ofbase materials and coatings.

There also exist numerous different deposition techniques based onspraying methods with different names such as Thermal Spray, HighVelocity Oxide Fuel (HVOF), Plasma Spray, Combustion Chemical VaporDeposition (CCVD); however, the underlying method is the same. Thecoating is sprayed onto a substrate and the material is fed into thenozzle or “flame” from either a rod, wire, stock, powdered material,liquid or gas. Spray techniques are most often used to coat details andare not suitable for roll-to-roll coatings, with high requirements onclose tolerances and high productivity.

Another method to coat a substrate is by hot dipping of the product intoa molten metal. Hot dipping is generally carried out with coatings thathave a low melting point, e.g., zinc, etc. For coatings with highermelting points, such as nickel and copper, the temperature of the moltenmetal is so high that it will often affect the substrate material in anegative way. Further, to have an accurate process control of suchmolten bath allowing close tolerances on layer thickness, is difficult.

Electroplating is an electrochemical process in which the coating isachieved by passing an electrical current through a solution containingdissolved metal ions and the metal object to be plated. The metalsubstrate serves as the cathode in an electrochemical cell, attractingmetal ions from the solution. Ferrous and non-ferrous metal substratesare plated with a variety of metals, including aluminum, brass, bronze,cadmium, copper, chromium, iron, lead, nickel, tin, and zinc, as well asprecious metals, such as gold, platinum, and silver.

As the substrate acts as a cathode in the process and thereby attractsthe ions in the solution, it is difficult for flat products to obtain aneven layer distribution. Local variations in current density will createan inhomogeneous deposition rate. A well-known problem is the “dog bone”effect that means that the thickness of the coating is often highertowards the edges of a coated strip. Further, the method ischaracterized by not being environmentally friendly as it involveselectrolytes and costly wastewater treatment. Electrochemical methodsand dipping methods also have the disadvantage that if a single sidedcoating is requested, the surface that shall remain uncoated has to bemasked in some way prior to the coating. The masking then has to beremoved subsequent to the coating operation.

There are also some vapor deposition methods that can be used fordepositing metals. Most methods are batch-like processes, but there arealso some continuous processes. One example of a roll-to-roll methodmaking use of electron-beam deposition is disclosed in WO 98/08986,which describes a method of manufacturing ferritic stainlessFeCrAl-steel strips, by bringing about an aluminum coating of asubstrate material in a roll-to-roll process. However, the methoddescribed in this patent application is optimized for a product suitablefor use in a high temperature corrosive environment, thus requiring amaterial with a good high-temperature strength and also a goodhigh-temperature corrosion resistance, i.e., oxidation resistance.Moreover, this patent application suggests that a homogenizationannealing at a temperature of 950-1150° C. is made in connection to thecoating, in order to have the aluminum evenly distributed in theferrite. This means that the final product in this case is not a coatedproduct with an aluminum layer on the surface. Hence, it is rather aFeCrAl strip product with a uniform distribution of the alloyingelements, including also aluminum. Further, this means that there are nospecial requirements on an oxide free interface and on a good adhesionof the layer.

Thus, all these conventional methods are suffering from differentdisadvantages, which means that there is a need for a development of anew product combining good mechanical properties with excellentelectrical characteristics and narrow geometrical tolerances.

All processes based on batch-type production will always increase thecost and it is therefore essential that the production will be by aroll-to-roll process to decrease the cost.

Therefore, it is a primary object of the present invention to provide aflexible metallic product with tailor-made physical and mechanicalcharacteristics suitable for further processing that may be exemplifiedby, but not limited to, blanking, bending, drilling, heat treatment etc.

Yet another object of the present invention is to provide a flexiblestrip product, for springs and other products, that requires a goodelectrical conductivity, made from a single- or multilayered metallicstrip that is inexpensive and which may be produced in a continuousroll-to-roll process.

These and other objects have been attained in a surprising manner bycreating a coated steel product with the features according to thecharacterizing clause of claim 1. Further preferred embodiments aredefined in the dependent claims.

BRIEF DESCRIPTION OF THE INVENTION

Thus, the above objects and further advantages are achieved by applyingone or more thin continuous, uniform, electrically conductive layer(s)of a metal, such as nickel, silver, tin, molybdenum, copper, tungstengold or cobalt, on the top of a metal stainless strip serving assubstrate. The coating may be done on one or both sides of the substratestrip. The metal layer should be smooth and dense and have a goodadhesion in order to allow for further processing without the risk offlaking or peeling. The final product, in form of a high strength stripsteel with one or two electrically conductive surfaces, is suitable foruse in electrical devices, in gaskets for electromagnetic shielding orfor any other purpose, where a high strength material with a low contactresistance in the interface between the product according to theinvention and its contact point is requested.

The coated layer is deposited by means of the previously known methodelectron beam evaporation (EB), in a roll-to-roll process, to an evenlydistributed layer with a thickness of preferably less than 15 μm. Thesubstrate material should be a stainless steel with a Cr content above10% (by weight) and with a strip thickness of usually less than 3 mm.The substrate material should have a tensile strength of at least 1000MPa, which can be achieved by cold deformation or by thermal treatmentsuch as hardening from high temperature or by precipitation hardening atlower temperatures. As a first step, the roll-to-roll process may alsoinclude an etch chamber, in order to remove the oxide layer thatotherwise normally is present on a stainless steel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic cross-section of a first embodiment of thepresent invention, i.e., a substrate strip 2 with a coating of anelectrically conductive layer 1, 3 on one or both surfaces. If thesubstrate is coated on both surfaces, then the coatings may be of thesame composition, or if so desired, of different compositions. Also thethickness of the coating may be the same or different for the twosurfaces.

FIG. 2 shows a schematic cross-section of a second embodiment of thepresent invention, i.e., a substrate strip 2 with coatings of multiplelayers (1,3,4 and 5,6, respectively) on one or both surfaces.

FIG. 3 shows schematically a production line for the manufacturing of acoated metal strip material according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The final product, in the form of a metal coated strip material, issuitable for the use as load carrying parts that also are characterizedby providing a low contact resistance at the interface. Examples of suchapplications are connectors and switches. By applying a given force onthe spring, it will contact a surface and thus close an electricalcircuit. At the point of contact, where the current is transferred, itis important that the contact resistance is low. Stainless steel is anincreasingly used material for spring applications. This is due to theattractive combination of high mechanical strength and good formability,allowing forming also rather complex spring geometries. High strengthstainless spring steels have in general superior mechanical propertiescompared to non-ferrous materials. In the context of spring properties,especially the fatigue and relaxation resistance of high strengthstainless steel are crucial for a long lasting spring with a constantforce throughout its service life. However, stainless steel ischaracterized by a passive film on the surface. This film consists ofchromium oxide and has a significantly lower electrical conductivitythan the steel itself. As a reference value, a stainless steel has anelectrical resistivity of 80-90×10⁻⁸ Ωm, depending on the tensilestrength. However, at the surface, the oxide (Cr₂O₃), has a resistivityof approximately 1.3×10¹¹ Ωm. If an oxide film is present at theinterface between two conductive surfaces, a drop in conductivity willoccur. This will decrease the efficiency in current circulation in thecircuit and thus decrease the performance.

To eliminate the problem of low conductivity in high strength stainlesssteel, at least one of the strip surfaces is coated with a metal layerthat is less prone to form an oxide film at the surface. The coatedlayer will thus allow for an oxide free surface at the contact point,whereby the drop in electrical conductivity at the interface is avoided.Depending on the requirements, the coating may be of different metals.Silver, copper, nickel, cobalt, gold, tungsten, tin and molybdenum areall metals with a good electrical conductivity that may be deposited onthe surface by the method according to the invention. It is also ofvital importance that the coating is homogeneously distributed on thesurface and is not too thick compared to the substrate thickness. Athick or an uneven layer will affect the spring properties, as thebending force is proportional to the thickness of a rectangular sectionraised to the third power. The thickness of the layer is thereforepreferably max 10% of the substrate thickness. Moreover, the thicknessof each coating layer is preferably maximally 15 μm, typically 0.05-15μm, preferably 0.05-10 μm and even more preferably 0.05-5 μm. Ifmultiple layers are to be deposited, then the total summarized thicknessof the coatings should not exceed 20% of the total thickness of thecoated strip. The thickness tolerance of the coated layer according tothe invention is very good. The variation in thickness of and withineach layer should not exceed +/−20% of the nominal thickness of saidlayer. More preferably, the thickness variation should be maximum +/−10%of the nominal thickness within each layer.

The coating should show a good adhesion to the substrate and thus makesubsequent manufacturing possible. The product according to theinvention shows an excellent adhesion between the coating and thesubstrate. This is achieved by a pre-treatment operation of thestainless strip by means of an ion etching in vacuum prior to thedeposition of the coating on the substrate. This allows for ametal-metal contact with an oxide free interface that will give aproduct that may be bent, blanked, slit or deep-drawn, the only limitbeing set by the ductility of the substrate material.

The Substrate Strip to be Coated

The material that shall be coated should have a good general corrosionresistance. This means that the material must have a chromium content ofat least 10% by weight, preferably minimum 12% or more preferablyminimum 13% or most preferably minimum 15% chromium. Further, thematerial must be alloyed in a way that allows for a high tensilestrength of at least 1000 MPa, more preferably a minimum of 1300 MPa oreven more preferably minimum 1500 MPa, or most preferably a minimum of1700 MPa. The mechanical strength may be achieved by cold deformationsuch as for steels of the ASTM 200 and 300 series, or by thermalhardening as for hardenable martensitic chromium steels. Other suitablesubstrate materials are precipitation hardenable (PH) steels of type13-8PH, 15-5PH, 17-4PH or 17-7PH. Yet another group of suitablesubstrate materials are stainless maraging steels that are characterizedby a low carbon and nitrogen containing martensitic matrix that ishardened by the precipitation of substitutional atoms such as copper,aluminum, titanium, nickel etc.

The Conductive Layer(s)

The material to be coated in the form of a thin layer film on thesubstrate surface should be characterized by a good electricalconductivity at room temperature, a thermodynamic stability againstoxide formation and a suitable modulus of elasticity. Thecharacteristics of the suitable elements are listed below.

Silver has a very low electrical resistivity, approximately 1.47×10⁻⁸Ωm, at room temperature. The free energy for oxide formation for Ag₂O atroom temperature is approximately ΔG=−10.7 kJ which makes silversignificantly more stable against oxidation compared with the formationof Cr₂O₃, as in stainless steel. As a reference value, Cr₂O₃ has a freeenergy at room temperature of approximately ΔG=−1050 kJ. Silver has amodulus of elasticity of approximately 79000 MPa that can be compared tothe 180,000-220,000 MPa for different steel types. Silver is howeverrelatively expensive and sometimes cheaper alternatives are required.

Copper has a low electrical resistivity of approximately 1.58×10⁻⁸ Ωm, amodulus of elasticity of approximately 210,000 MPa and a free energy ofΔG=−145 kJ and ΔG=−127 kJ for the formation of Cu₂O and CuOrespectively. This combination of properties makes also copper asuitable coating in the product according to the invention.

Nickel has a low electrical resistivity of approximately 6.2×10⁻⁸ Ωm, amodulus of elasticity of 200,000 MPa and a free energy of approximatelyΔG=−213 kJ for the formation of NiO.

Gold has an electrical resistivity of approximately 2×10⁻⁸ Ωm, a modulusof elasticity of 80,000 MPa. Gold is also extremely stable againstoxidation. This makes gold in many applications most suitable as anelement for conductive coatings. However, gold is expensive andalternatives are always looked for due to the high alloy cost as well asre-cycling costs.

Molybdenum has a low electrical resistivity of approximately 5.3×10⁻⁸Ωm, a modulus of elasticity of 329,000 MPa and a free energy ofapproximately ΔG=−668 kJ for the formation of MoO₃ and ΔG=−533 kJ forthe formation of MoO₂.

Cobalt has a low electrical resistivity of approximately 6.24×₁₀ ⁻⁸ Ωm,a modulus of elasticity of 209,000 MPa and a free energy ofapproximately ΔG=−241 kJ for the formation of CoO.

Tungsten has a low electrical resistivity of approximately 5.3×10⁻⁸ Ωm,a modulus of elasticity of 360,000 MPa and free energies ofapproximately ΔG=−534 kJ and ΔG=−764 for the formation of WO₂ and WO₃,respectively.

Tin has an electrical resistivity of approximately 10×10⁻⁸ Ωm and amodulus of elasticity of 50,000 MPa. The free energy to form SnO isapproximately ΔG=−534 kJ at room temperature. Tin is also a relativelysoft metal and is easily deformed at the point of contact and may bythis generate a larger contact area at the interface. This may beutilized, e.g., in gasket springs for electromagnetic shielding.

Description of Coating Method

Advantageously, the coating method is integrated in a roll-to-roll stripproduction line. In this roll-to-roll production line, the firstproduction step is an ion-assisted etching of the metallic stripsurface, in order to achieve good adhesion of the first layer. Theconductive layer is deposited by means of electron beam evaporation (EB)in a roll-to-roll process. The formation of multi-layers can be achievedby integrating several EB deposition chambers in-line (see FIG. 3).

PREFERRED EMBODIMENT OF THE INVENTION

Two examples of embodiments of the invention will now be described inmore detail. One example is based on a silver coating on a ASTM 301-typeof steel with a chemical composition of max 0.12% C, max 1.5% Si, max 2%Mn, 16-18% Cr and 6-8% Ni with balance Fe and residual elements that arepresent according to the metallurgical method used. The second exampleis a nickel coating on a modified ASTM 301-type of steel with a chemicalcomposition of max 0.12% C, max 1.5% Si, max 2% Mn, 16-18% Cr and 6-8%Ni, 0.5-1.0% Mo with balance Fe and residual elements that are presentaccording to the metallurgical method used.

Firstly, the substrate materials are produced by ordinary metallurgicalsteel-making to a chemical composition as exemplified above. They arethen hot rolled down to an intermediate size, and thereafter cold-rolledin several steps with a number of recrystallization steps between saidrolling steps, until a final thickness of about 0.02-1 mm and a width ofmaximum 1000 mm are attained. The surface of the substrate material isthen cleaned in a proper way to remove all oil residuals from therolling.

Thereafter, the coating process takes place in a continuous processline, starting with decoiling equipment. The first step in theroll-to-roll process line can be a vacuum chamber or an entrance vacuumlock followed by an etch chamber, in which ion-assisted etching takesplace in order to remove the thin oxide layer on the surface of thestainless substrate material. The strip then enters into the E-beamevaporation chamber(s) in which the deposition of the desired layertakes place. A metal layer of normally 0.05 up to 15 μm is deposited,the preferred thickness depending on the application. In the twoexamples described here, a thickness of 0.2-1.5 μm is deposited by usingone E-beam evaporation chamber.

After the EB evaporation, the coated strip material passes through theexit vacuum chamber or exit vacuum lock before it is coiled on to acoiler. The coated strip material can now, if needed, be furtherprocessed by, for example, rolling or slitting, to obtain the preferredfinal dimension for the manufacturing of components.

The final product as described in the two examples, i.e., a 0.05 mmthick strip of ASTM 301 with a single sided 1.5 μm Ag-coating and a 0.07mm thick strip of ASTM 301, modified with a single sided 0.2 μmNi-coating, have a very good adhesion of the coated layer and are thussuitable to be used in subsequent manufacturing of components for highstrength applications, e.g., for springs. The good adhesion of thelayers is tested according to standard tensile testing. From a substratematerial of a stainless steel strip that has been coated with a thincovering layer so as to produce a coated strip product in accordancewith the present invention, tensile test specimens are producedaccording to standard. Tensile testing of 4 specimens, for exampleaccording to EN 10002-1, is thereafter carried out until fracture. Aftertesting, the fractured part of the specimen is investigated in anoptical microscope with a magnification of 50 times. Beside the actualfracture from testing, no signs of flaking, peeling or any other damageof the coated layer has been observed in any tested specimen. Theresults from this test are presented in Table 1. TABLE 1 Mechanicalproperties and adhesion of layer. Proof Proof Tensile Visual Thick-strength strength stregth examination ness, Rp 0.05%, Rp 0.2%, Rm, at 50times Sample Mm MPa MPa MPa magnification 301 + Ni 0.07 1659 2108 2120No peeling or flaking, 301Mod + 0.05 1445 1920 1945 No peeling or Agflaking

The roll-to-roll electron beam evaporation process referred to above isillustrated in FIG. 3. The first part of such a production line is theuncoiler 13 within a vacuum chamber 14, then the in-line ion assistedetching chamber 15, followed by a series of EB evaporation chambers 16,the number of EB evaporation chambers needed can vary from 1 up to 10chambers, this to achieve a multi-layered structure, if so desired. Allthe EB evaporation chambers 16 are equipped with EB guns 17 and suitablecrucibles 18 for the evaporation. After these chambers, comes the exitvacuum chamber 19 and the recoiler 20 for the coated strip material, therecoiler being located within vacuum chamber 19. The vacuum chambers 14and 19 may also be replaced by an entrance vacuum lock system and anexit vacuum lock system, respectively. In the latter case, the uncoiler13 and the coiler 20 are placed in the open air.

1. A coated stainless steel strip product with a dense and evenlydistributed layer on one side or both sides of said strip wherein saidlayer consists essentially of one or several of the metals gold, copper,nickel, molybdenum, cobalt, silver, tin or tungsten, that the wherein athickness of said layer is preferably maximally 15 μm, wherein atolerance of said layer is maximally +/−30% of the layer thickness,wherein a Cr content of the steel strip substrate is at least 10%, andwherein the layer has an adhesion to the strip that the coated steelstrip, under uniaxial stretching to fracture by tensile testing, doesnot show any tendency to peeling or flaking.
 2. Product according toclaim 1 wherein the thickness of the strip substrate is between 0.015 mmand 3.0 mm.
 3. Product according to claim 1, wherein said strip includesa substrate of austenitic stainless steel, or duplex stainless steel, orhardenable martensitic chromium steel, or precipitation hardenablestainless steel, or maraging steel with a minimum tensile strength of1000 MPa in the cold rolled or heat treated condition.
 4. Productaccording to claim 1, wherein the layer has a multi-layer constitutionof up to 10 layers.
 5. Product according to claim 4 wherein eachindividual layer has a thickness of between 0.05 to 15 μm.
 6. Productaccording to claim 4 wherein each individual layer has a thickness ofbetween 0.05 to 11 μm.
 7. Product according to claim 4 wherein eachindividual layer has a thickness of between 0.05 to 5 μm.
 8. Productaccording to claim 5, wherein the layer has a multi-layer constitutionof individual layers of different metallic coatings.
 9. Productaccording to claim 8, wherein the layer consists of alloys of at least 2elements selected from the group consisting of gold, copper, nickel,molybdenum, cobalt, silver, tin and tungsten.
 10. A product according toclaim 1, wherein the product is suitable for use in load carryingapplications where a low contact resistance at the surface isadvantageous.
 11. A product according to claim 1, wherein the product issuitable for the production and use of spring elements is in switches,connectors, or metallic domes.
 12. Method of manufacturing a coatedstainless steel strip product according to claim 1, comprising producingthe coated stainless steel strip product in a continuous roll-to-rollprocess included in a strip production line using electron beamevaporation comprising an etch chamber in-line.
 13. Product according toclaim 8, wherein the different metallic coatings are selected from thegroup consisting of Ag, Ni, Mo, Co, Au, Mo, W and Sn.